56 SUNFLOWER. [CH. IV 



razor, and if the stem is split longitudinally it will be 

 obvious that each dot corresponds to a rope or strand of 

 fibrous tissue running down the stem. Each of these 

 dots is a vascular bundle, and is seen in fig. 22 to be made 

 up of a mass of vascular tissue (ph, oc) and of a patch of 

 fibres (f). These fibres 1 were, until recently, called 

 bast fibres ; they are now usually described as pericycle- 

 fibres because they originate in the region known as the 

 pericycle, which forms the external limit of the central 

 cylinder. 



It should be noted that between the vascular bundles, 

 avenues of parenchyma run towards the cortex : these 

 radiating paths by which, except for the pericycle, pith 

 and cortex are joined, are known as medullary rays, and 

 will be seen later on to be of great importance. 



Each vascular bundle consists of three kinds of tissue, 

 as seen in figs. 22, 23, 24. 



I. Xylem, nearer the pith. 



II. Phloem, nearer the cortex. 

 in. Cambium, between the two. 



The arrangement differs strikingly from that of the 

 root where free strips of xylem alternated with free strips 

 of phloem ; here each vascular strand contains both xylem 

 and phloem. (See Preface on the word stele.) 



1 There seems to me no substantial inaccuracy in using the term 

 bast for the pericycle fibres as well as for the hard elements of the phloem. 

 The word becomes purely descriptive and does not assert a common 

 origin of the tissues to which it is applied. 



